The present invention relates to a motor velocity control system employing feedback to maintain actual motor rotational velocity at or near a desired rotational velocity and, more particularly, to such a motor control system which implements a clock rate control motor rotational velocity system by advantageously apportioning the required functions between a relatively small, slow speed microprocessor, and a plurality of discrete circuit elements in the form of analog and digital logic devices.
The above identified, commonly assigned Alley and Weischedel Application Ser. No. 109,579 describes a motor velocity control concept particularly intended, but not so limited, for application in a direct drive domestic clothes washing machine wherein, through suitable input control signals, a DC motor is controlled as to both rotaional velocity and direction to effect desired agitator motions.
More particularly, in the above-identified commonly assigned Alley and Weischedel Application Ser. No. 109,579, desired motor rotational velocity is commanded by an input clock rate, which may be variable, rather than by a voltage or current signal. A sensor is provided for determining actual rotor position, and actual rotor position is periodically compared with instantaneous desired rotor position as indicated by the time of occurance one of the input signals or pulses. Depending upon the results of this periodic comparision, a dynamically established active current limit is modified. Specifically, if motor rotor actual position leads desired position, the current limit is reduced. If, on the other hand, actual motor position is lagging desired motor position, the current limit is increased.
As pointed out in that application, this control action occurs simultaneously with conventional winding commutation taking place under the control of a commutation sequencer periodically advanced in conventional fashion as the rotor reaches each of a plurality of predetermined positions.
The particular system described in the Alley and Weischedel Application Ser. No. 109,579 is a so called hard wired implementation comprising a plurality of integrated circuit devices such as CMOS digital logic devices, and analog comparators and operational amplifiers.
It is recognized that, with the widespread use of relatively low cost microprocessors in a wide variety of products, microprocessor control of a device, such as a clothes washing machine, which includes a velocity controlled electronically commutated motor is a likely and desirable end. In particular, the optional cycle sequencing aspects of the control system of such a device readily lend themselves to microprocessor control. Moreover, it has been recognized that different agitator velocity (or instantaneous position) profiles, other than the commonly employed sinusoidal profile, are desirable or beneficial in clothes washing. An agitator velocity motion profile may be described as a waveform having a frequency in the order of one or two hertz, and may be a simple sinusoid, or some other waveform such as triangular or trapesoidal.
It is envisioned that such a velocity profile waveform might conveniently be stored in a memory, such as a read only memory (ROM) as a plurality of discrete points along a curve. For example, it has been determined that in the order of sixteen points are adequate to define a clothes washing machine agitator velocity profile which provides sufficiently smooth operation when advancing from one point to the next.
As pointed out in the above referenced commonly assigned Alley and Weischedel Application Ser. No. 109,579, a preferred method of periodically determining actual rotor position is that which is described in the commonly assigned Wright U.S. Pat. No. 4,162,435. Briefly, in the approach described in the Wright patent, induced voltage is sensed across an unenergized motor winding, and that voltage sample is integrated and compared to a reference voltage. When the integral exceeds the reference voltage, the system recognizes that a predetermined rotor position has been reached, and the commutation sequence proceeds to the next step.
It will be appreciated that a microprocessor based computer control system may readily be devised to implement all of the required functions of a clock rate control motor velocity control system, including the integration and integration limit functions. However, for such a system to operate in real time to continuously provide commutation and update the current limit as required during motor rotation would require a relatively fast microprocessor, with attendant expense.